1. Field of the Invention
The present invention relates to the field of etching of film layers on substrates, including the etching of film layers formed on semiconductor substrates and on insulative substrates, such as glass substrates, to selectively remove portions of the film layer. More particularly, the present invention has application to the etching of film layers on large planar surfaces such as those encountered in the fabrication of large flat panel displays.
2. Background of the Art
Selective etching of film layers on semiconductor substrates is well known. For example, U.S. Pat. No. 4,367,672, Wang, et al., fully incorporated herein by reference, discloses methods of using a plasma to selectively etch holes or trenches in a film layer on a semiconductor substrate. Currently used semiconductor substrates are typically circular, having a diameter of no more than 200 mm, a thickness of less than 0.5 mm, and a mass of no more than approximately 60 g. Because of the relatively small size and weight of the semiconductor substrate, relatively straightforward etch chamber configurations may be used to provide the etch process environment to selectively etch a film layer on the substrate, and relatively straightforward wafer handling equipment may be used to load the substrates into processing chambers.
The process for manufacturing flat panel displays uses many of the same processes used to fabricate semiconductor devices. The manufacture of the flat panel display begins with a clean glass substrate. Transistors are formed on the flat panel using film deposition and selective etching techniques similar to those described in the aforementioned Wang, et al. patent. Sequential deposition, photolithography and selective etching of film layers on the substrate creates individual transistors on the substrate. These devices, as well as metallic interconnects, liquid crystal cells and other devices formed on the substrate are then used to create active matrix display screens on the substrate to create a flat panel display in which display states are electrically created in the individual pixels.
Although the flat panel display is typically manufactured using the same processes as those used in semiconductor device fabrication, the glass used as the flat panel display substrate is different than a semiconductor substrate in several important respects that affect its processing. In semiconductor fabrication, individual devices are formed on the wafer, and the wafer is diced to form multiple individual integrated circuits. Thus, the creation of some defective devices on the semiconductor wafer is tolerated, because the die bearing these defective devices are simply discarded once the substrate is sawn into individual integrated circuits. On the flat panel display, individual defective devices must not be removed. Therefore, the number of defective devices created on the flat panel substrate must approach zero. If a substrate is sufficiently large to process multiple displays on a single substrate, a defect in any one of the flat panel displays being formed on the flat panel substrate renders that display defective. Additionally, the glass substrates are typically substantially larger than the largest semiconductor wafers, and the coefficient of heat transfer of the glass substrate is approximately 100 times less than the coefficient of heat transfer of the semiconductor substrate.
In semiconductor processing, in particular etch processing, the process environment transfers substantial energy into the substrate, and this raises the temperature of the substrate if the energy is not distributed away from the surface of the substrate and/or removed from the substrate at the same rate as it enters the substrate. In semiconductor substrate processing, the substrate temperature is maintained at a desired level by balancing the energy transferred into the substrate by the process environment with the combination of the capacity of the substrate to distribute the heat away from the surface and the rate at which heat is transferred from the substrate into the substrate support member. As the substrate is heated by the process environment, the temperature of the substrate outer surface is raised by the energy transferred into the substrate. As the etch process continues, the heat conducts into the substrate to raise the temperature in the remainder of the substrate. A portion of this heat is eventually conducted into the support member. By balancing the total energy transferred into the substrate during the etch process, and the rate of energy transfer into the substrate, the temperature of the substrate surface may be maintained below the resist breakdown temperature of 120 degrees Celsius.
Where a substrate is received on a support member but not firmly chucked thereto, the rate of heat transfer from the substrate into the support member is relatively small because the vacuum process environment substantially prevents conductive heat transfer between the substrate and the support member in areas where the support member and the substrate are not in intimate contact. Therefore, where the substrate is simply mechanically coupled to the support member, the power must be maintained at relatively low levels so that the energy input into the substrate does not exceed the combination of the rate of heat transfer from the substrate into the support member and the diffusion of the heat energy within the substrate. However, because etch rate and power density are approximately directly proportional, these low energy levels limit throughput of substrates through the chamber because longer process times are required to effect a desired etch. To increase heat transfer, and thus throughput, the substrate may be clamped to the support member. This method of controlling the temperature of semiconductor substrates may also be adapted to the processing of flat panel display substrates.
In flat panel display substrate processing, the large, rectangular, glass sheet used as the substrate is heavy and bulky but nonetheless fragile, and cannot be easily manipulated between the horizontal and vertical planes. Therefore, the plasma etch processes used to selectively etch the film layer on a flat panel substrate are typically performed with the substrate located in a horizontal position, because it is easier to handle the substrate in process if it is loaded into a process chamber in the horizontal plane.
A flat panel display substrate etched in a horizontal position, however, is prone to the formation of defects. In the typical etch process for a horizontal flat panel substrate, the etch plasma is maintained within the chamber enclosure above the substrate. Therefore, as the film layer is etched, contaminants in the enclosure fall by the force of gravity onto the substrate, and contaminants may also be electrostatically attracted to the substrate. Each of these contaminants above a threshold size will produce a defect in a flat panel formed on the substrate.
The process chemistries used to create flat panel displays using amorphous silicon layers also contribute to the creation of defective displays. In flat panel display substrates, the holes or trenches etched in the film layers must typically have tapered walls. Typically, where the layer being etched is amorphous silicon, fluorine based chemistries are used for etching. To provide the tapered side walls using a fluorine based chemistry, oxygen is introduced into the etch chemistry. As the fluorine based chemistry etches the film layer, the oxygen continuously etches the edge of the resist to continuously reduce the width of the resist and thus increase the width of the area of the film being etched to create the desired tapered walls. However, the oxygen used to etch the edge of the resist also forms particle contaminants in the chamber, which, if received on the substrate, will cause a particle defect.
Therefore, there exists a need in the art for substrate processing equipment that will allow etching of large substrates, such as for flat panel displays, with maximum throughput and minimal process variation.